The present invention relates to an adsorbent layer made up of an assembly of modules of structured adsorbent, to an adsorber comprising at least one such layer and to a method employing such an adsorber for the purposes of ridding a fluid of at least one of it impurities.
In general, a method referred to as an adsorption method allows one or more gas molecules to be separated from a gaseous mixture containing them by exploiting the difference in affinity of a given adsorbent or, where appropriate, of several adsorbents, for these different gas molecules.
The affinity an adsorbent has for a gas molecule is dependent on the structure and composition of the adsorbent and on the properties of the molecule, notably its size, its electronic structure and its multipole moments.
An adsorbent may for example be a zeolite, an active charcoal, an activated alumina, a silica gel, a carbon-containing molecular sieve, a metallo-organic structure, an alkali-metal or alkaline-earth oxide or hydroxide, or a porous structure containing a substance capable of reacting reversibly with one or more gas molecules, such as amines, physical solvents, metallic sequestering agents, metal oxides or hydroxides for example.
Conventional adsorbent materials take the form of particles and are used in vessels (reactors) referred to as “adsorbers”. The most common place adsorber geometries are cylindrical adsorbers with a vertical axis, with a horizontal axis, and radial adsorbers.
Said standard adsorbers take the form of beads, generally of diameters ranging from 0.5 to 5 mm, in the form of crushed adsorbents on a millimeter scale (generally measuring from 0.5 to 5 mm likewise), in the form of adsorbent pellets of diameters ranging from 0.5 to 6 mm and of length shorter than 1 cm. There are also a few extruded adsorbents of more complex shape such as trilobal adsorbents made up of three cylinders joined together, for example.
These adsorbents are tipped loose into the adsorbers and constitute a bed.
Several layers of adsorbents of different natures can thus be placed one on top of the other (or side by side in the case of a radial adsorber). In operation, the fluid passes through these successive beds by passing around said particles and the most adsorbable constituents are stopped preferentially within the adsorbent.
The use of the smallest particles generally makes it possible to improve the adsorption dynamic and thereby the efficiency of the method, but the counterpart to that is that they create large pressure drops in the fluid phase.
To counterbalance this effect, use is made of adsorbers that have a large passage cross section for the fluid, such as the horizontal-axis cylindrical adsorbers or the radial adsorbers mentioned hereinabove.
However, if seeking to improve the pressure drop and/or dynamics still further, this technology leads to adsorber geometries that are industrially unfeasible.
This is, for example, what happens where there is a wish to process large gas flow rates at low pressure, such as when capturing CO2 from effluents at atmospheric pressure or when rapid cycles are to be performed, particularly PSA cycles.
For this reason, the structured adsorbent concept has recently been developed, as structured adsorbents, as opposed to particulate conventional adsorbents, have a more complex geometry, with dimensions substantially greater than one centimeter and offer the fluid a larger or easier passage. As opposed to particulate adsorbents (beads, pellets, crushed adsorbents) of dimensions smaller than 1 cm, that are tipped loose into an adsorber with the fluid circulating around the particles, structured adsorbents are solid materials of dimensions ranging from a few centimeters to a few meters and have passages that are free to gas, such as monoliths, foams or cloths. This type of adsorbent is notably described in the F. Rezaei, P. Webley document Separation and Purification Technology 70(2010) 243-256. Structured adsorbents (in comparison with granular adsorbents) have the special feature of allowing very good dynamics and very small pressure drops without having a known attrition limit. While these structures are currently far more expensive than granular adsorbents, their economic benefit as a full replacement for granular beds may prove decisive if it is accompanied by an appreciable improvement in pressure drop and/or a significant reduction in the cost of constructing the adsorber by reducing the volume of adsorbent or simplifying the construction.
The structured adsorbent used for preference is a contactor with parallel passages.
Contactors with parallel passages means a subgroup of structured adsorbents in which the fluid passes along channels, the walls of which contain the adsorbent, which channels are essentially free of obstacles and allow the fluid to circulate from an inlet of the contactor to an outlet thereof. These channels may be straight, connecting the inlet of the contactor to the outlet directly or may have changes in direction.
As it circulates, the fluid is in contact with at least one adsorbent present at said walls.